1. Field of the Invention
The present invention relates to a method of fabricating an optical fiber preform that generates gaseous effluents containing halogenated compounds. The invention relates more particularly to treating the effluents in order to recover them or eliminate them. It also encompasses plant for implementing the method.
2. Description of the Prior Art
The chemical vapor deposition (CVD) process is used in the fabrication of preforms from which optical fibers are produced. A hollow silica tube is mounted on a glassmaker""s lathe and a mixture of several gases is injected into it, the mixture including gaseous halogenated compounds such as silicon tetrachloride SiCl4 and germanium tetrachloride GeCl4. A torch sweeps the whole length of the tube, from one end to the other, first in one direction and then in the opposite direction, which represents one pass. Because of the effect of the temperature, the halogenides are oxidized, forming soot that is deposited on the internal walls of the tube and is finally vitrified. Several successive layers of glass are deposited on the inside of the tube in this way. The purpose of the first pass is to allow the temperatures of the various systems present to equalize. During this first pass, the gaseous halogenides are not directed into the tube, but are instead evacuated directly to an outlet. Halogenides are regarded as pollutants, and it is not acceptable to release them into the atmosphere. For ecological reasons, halogenides must therefore be eliminated or recovered. This is usually done by means of a soot box, which is a container adapted to collect the particles that are formed when the halogenides are brought into contact with ambient moist air. This deposition continues in the network of pipes. Lumps of solid particles are formed, which are in large part the cause of blocking of the pipes.
A method known from the document FR-2 522 278 is used to treat the gaseous waste to recover germanium. The method includes a step of passing the gaseous effluents into a washer. The effluent is therefore brought into contact with an aqueous solution. A solution is used that is sufficiently basic for the chlorides, and in particular germanium tetrachloride GeCl4, to be trapped by hydrolysis. For cost reasons, the germanium is then recovered by adding to the aqueous medium a precipitation agent containing a multivalent cation. The document FR-2 773 724 proposes using magnesia MgO as the precipitation agent.
An object of the present invention is to propose a reduced maintenance optical fiber preform fabrication method. To be more specific, the invention proposes a method of treating gaseous waste that reduces the risk of blocking of the pipes and facilitates recycling of the effluents.
The present invention provides a method of treating gaseous waste from an optical fiber preform fabrication unit containing halogenated compounds, the method including a step of forming a liquid effluent from the gaseous waste by condensing the gaseous waste.
Condensation is preferably obtained by cooling. For example, condensation is effected by circulating the gaseous waste in a refrigerating device such as a heat exchanger. Condensation could instead be achieved by compression. After condensation, the liquid effluent is collected in a collector container and then treated.
The plant for implementing the method according to the invention includes a condenser and a container for collecting the condensate. The condenser is preferably a refrigerating device.
The plant preferably further includes a soot box. The soot box causes the residual chlorides to react by bringing them into contact with moisture. All along the fabrication line, and up to the point at which they enter the soot box, the chlorides are in contact only with dry gases. When they reach the soot box, they are mixed with moist ambient air and form solid particles. The function of the soot box is to provoke solidification and to collect the products thereof. Given the large volume of gas to be treated, the first pass constitutes a separate operation using the plant. A plurality of fabrication lines can therefore share the plant and operate simultaneously.
The present invention has the advantage of recovering a large proportion of the waste halogenides, and consequently of improving the recovery of germanium in particular. Apart from recycling the halogenated substances, reducing the volume of effluent passing through the soot box considerably reduces soiling of the pipes and therefore eliminates the problem of backflow caused by clogging of the network and reduces plant maintenance. Finally, it is no longer necessary to remove waste accumulating in the soot boxes, which reduces the production cost.
The invention will be more clearly understood and other advantages and features will become apparent on reading the following description, which is given by way of illustrative and non-limiting example, and is accompanied by the appended drawings.